Thermoplastic, semi-rigid tube has a variety of uses such as for underground drainage or for carrying electrical wiring. For various applications it has been found that corrugated tube, either double walled or single walled has advantages and, more recently, the use of a ribbed wall tube has become widespread. The ribs of such tube are usually annular or helical ribs upstanding from the tube wall by a height appreciably in excess of the thickness of the tube. The use of such ribbed tube gives good compression strength in comparison with solid walled tubes but utilizing less weight than plastics materials.
Ribbed tube has significantly larger outside diameter than plan walled tube and if they are to be joined together, coupling members of large internal diameter are generally unsuitable for joining ribbed tube with plain walled tube. For joining ribbed tubes, it is necessary to provide sealing means which must fill the entire gap between the ribbed tube base wall and the coupling sleeve. If the ribs upstand to any great degree this gap may be extensive. For example, if ribs upstand from the base wall of the tube by 2.5 cm or more, the seal must effectively seal an annular gap of at least 2.5 cm in width. When the ribs are annular, the seal may be located between two ribs.
A coupling sleeve may be provided as an independent unit or it may be provided by an expanded or belled end of one tube which fits over the end of another tube. In either case difficulties arise in accurate fit and sealing. Moreover, when the coupling sleeve is a belled end of one tube, the degree of belling has to be very large in order to fit over the ribs of the other tube. Further problems arise when it is desired to couple ribbed tube and plain walled tube together. It may be seen that, if ribbed tube of large outside diameter is to be joined to plain walled or other tube of small outside diameter, then there is a very large gap to be sealed between the two tubes. Generally, coupling between plain walled tube and ribbed tube is not carried out because of the difficulties involved. However, where it cannot be avoided, it has been found necessary to use independent coupling sleeves for the purpose. One reason for this is that bells provided in the ends of tubes to fit over ends of adjacent tubes must be sufficiently large to fit over ribbed tube. Thus, the gap presented between plain walled tube and the large coupling bell at the end of ribbed tube is too large to be sealed by conventional sealing means.
Ribbed tube of the type having substantially raised ribs is usually made by means of apparatus of the travelling mold tunnel type. A tubular parison of plastics material is extruded from an extrusion nozzle into a travelling mold tunnel over a forming plug. The forming plug defines the inner tube surface and the travelling mold tunnel defines the outer tube surface. Thus, the parison is forced into the cavity between the plug and the tubular tunnel which moves axially over the plug. The tubular tunnel has a mold surface to form the ribs or other configuration on the final tube. There are various means of providing such a travelling mold tunnel but this is often comprised by two tracks of mold blocks which come together in cooperating pairs to form a tunnel on a forwardly moving run parallel and coaxial with the extrusion nozzle.
It is known to introduce at least one pair of mold blocks, known as cuff blocks, adapted to mold a cuff of different outer surface of the tube into the mechanism of the travelling mold tunnel so that such different surface will be cyclically introduced on the surface of the molded tube according to the length of the runs of mold blocks. Of course such differing pairs of mold blocks may be inserted into the mold train at whatever intervals are desired. Thus, more than one such pair may be present in the runs.
For example, European patent application No. 0,385,465 discloses a double-walled corrugated tube comprising a smooth inner tube and a corrugated outer tube. The outer tube and the inner tube are co-extruded and are connected to each other by thermal welding. The corrugated outer tube is made by means of an apparatus of the travelling mold tunnel type. Smooth walled mold blocks are introduced in the mold tunnel, in order to get tube sections with a smooth outer surface. The tube is cut off in the middle of the smooth walled portions so that they form the ends of the tube. One end is belled as long as the tube is warm, in order to fit to the other end of a second tube for forming a coupling.
On the other hand this document also discloses a corrugated outer tube and smooth inner tube one end portion of which is belled. Due to the expansion of the bell the corrugations of this portion of the tube is flattened. At the other end of the tube the corrugations are also flattened without changing the inner diameter of the inner tube. The flattening of the corrugations at the other end portion is necessary for reducing the outer diameter of the outer tube in order to adapt it to a standard size. The flattening is obtained for instance by heat-forming.
European patent application No. 0,291,136 discloses a thermoplastics material tube having solid ribs upstanding from an outer base wall surface, the ribs being of a height at least as great as the wall thickness of the tube, the tube having ends formed as co-operating smooth walled spigots and sockets for joining lengths of tubes together. The smooth portions of the tube are also formed by smooth walled mold blocks which are introduced in the travelling mold tunnel at the desired locations.
The forming of such a tube by an injection molding process however leads to the following difficulties. It is not known to have been possible to make smooth walled tube in a travelling mold tunnel due to transport difficulties of the parison of the mold. It has been so well known that a travelling mold tunnel tends to slip with respect to the tube being molding in it, that it is generally regarded in the industry that molding smooth walled tube in a travelling mold tunnel is not possible. Even the manufacture of relatively short sections of smooth walled tube in an otherwise ribbed tube which is formed in an travelling mold tunnel is believed to give difficulty due to transport difficulty of the smooth walled section, which is known to affect the quality of the resulting plastic.
The present inventor has tried to overcome this longstanding problem and to provide an essentially smooth walled coupling portion in a travelling mold tunnel without affecting the quality of the resulting plastic.
According to the invention there is provided a thermoplastics material tube having solid ribs upstanding from an outer base wall surface, the ribs being of a height at least as great as the wall thickness of the tube, the tube having a first end and a second end, the first end comprising a short length of tube in which the ribs are of reduced height less than the wall thickness of the tube and define a groove for seating a sealing ring and the second end being belled sufficiently to fit over the first end of another tube. Preferably the second end has an inner annular groove adapted to engage a sealing ring.
A sealing ring for sealing between the first end of one tube and a second end of another tube may be provided in said inner annular groove.
The ribs may be of any configuration but it is envisaged that the invention will be of practical use with either of annulary ribbed and helically ribbed tube.
Also according to the invention there is provided a method of coupling one ribbed tube with another ribbed tube, each tube having ribs of a height greater than the wall thickness of the tube, comprising forming an end of reduced rib height less than the wall thickness of the tube in said one ribbed tube, seating a sealing ring located in a groove there between forming a belled end in the other ribbed tube, the diameter of the belled section being such that the bell section fits over an end of reduced height of said one ribbed tube, and fitting the belled section over said end of ribbed tube height.
Preferably the method includes forming an annular groove in an inner wall of the belled section locating a sealing ring in the groove.